The present invention relates to an image forming device such as laser printers, photocopiers, facsimile machines, and more particularly to an electrophotographic type image forming device provided with sheet sensors.
A conventional electrophotographic type image forming device, such as a laser printer, has been disclosed, for example, in laid open Japanese patent application publication No. HEI-10-35941. In this type of image forming device, a toner image formed on a photosensitive drum is transferred to a sheet or another recording medium as the medium passes between a transfer roller and the photosensitive drum. Subsequently, the recording medium passes between a single heat roller and a single pressure roller, causing the toner to melt by heat and fixing the toner image to the surface of the recording medium. A feeder unit is provided for supplying the recording medium stacked in a sheet supply tray, one sheet at a time. The sheet of the recording medium is conveyed from the feeder unit to an image forming position at which the photosensitive drum opposes the transfer roller through a conveying path by a sheet supply roller, a plurality of intermediate conveying rollers, and a register roller provided along the conveying path.
When forming images on a plurality of recording mediums in succession, the start timing for supplying the second and subsequent sheets from the feeder unit is set as follows. If the trailing edge of the preceding sheet is too close to the leading edge of the following sheet, sheet sensors disposed along the conveying path from the feeder unit to a discharging unit may mistakenly detect a sheet jam if the leading edge of the following sheet is conveyed over a sheet sensor before the sheet sensor detects the trailing edge of the preceding sheet. To prevent the detection of such sheet jam and also to secure enough time for developing the image data, normally the start timing for supplying sheets is set such that an interval of a prescribed amount, for example approximately 60 mm, is provided between the trailing edge of the preceding sheet and the leading edge of the following sheet.
For supplying each one sheet from the feeder unit, a frictional separating mechanism is used for separating the one sheet from the remaining sheet stack. One such frictional separating mechanism employs a separation pad pressing against the sheet supply roller to generate friction for separating the sheets. However, sometimes the leading edges of the sheets in the feeder unit become uneven. When using such frictional separating mechanism, a following sheet can oppose the frictional force between the sheet supply roller and frictional separating pad and move in the conveying direction as the preceding sheet is conveyed due to attractive force between the preceding uppermost sheet and the subsequent lower sheet stacked in the sheet supply tray. In this case, the subsequent sheet stops with its leading edge shifted downstream in the conveying direction from a reference position at the nip area between the sheet supply roller and frictional separating pad when the sheet supply roller has stopped.
If the following sheet stops with its leading edge shifted too far downstream from the reference point at the nip area when the sheet supply roller stops, the required condition of maintaining a prescribed interval between neighboring sheets cannot be met. Hence, when the sheet supply roller is driven to rotate at a prescribed timing designed to maintain a prescribed interval between sheets, it is possible that the sheet sensors will detect a sheet jam as described above or that sufficient time is not allocated for developing the image data to be printed.
In view of the foregoing, it is an object of the present invention to provide an image forming device capable of forming images at appropriate positions on a recording medium, while maintaining a fast processing speed. This and other objects will be attained by an improved image forming device including an image forming unit, a feeder unit, a leading edge sensor, and a control unit. An image is formed on an image recording medium at the image forming unit. The feeder unit accommodates therein a stack of a plurality of image recording mediums. The feeder unit includes a sheet supply member that separates one image recording medium from a remaining stack, and feeds each one image recording medium toward the image forming unit in succession in a medium feeding direction with an interval between a trailing edge of a precedent image recording medium and a leading edge of a subsequent image recording medium. The leading edge sensor detects each leading edge of each image recording medium fed by the feeder unit. The lead edge sensor is communicated with the control unit, and the control unit controls a driving timing of the sheet supply member for controlling a feed start timing to feed the image recording medium based on the detection by the leading edge sensor.